Make sure to look carefully at corner cases, like max-load at 
min-input-voltage (typically causes max inductor current).

>From the app-note, I see there is a discrete NMOS transistor, and 
thankfully a single inductor (not transformer), so your modeling is easier. 
Be very careful about the voltage spikes across the NMOS device; you must 
never allow Vds to exceed the  max-spec value, which often derates at 
higher temperature.

There are a lot of devices with low Rds(on); when I say low, I'm talking 
below 50milliohms, but they often have higher gate-source capacitance and 
require higher gate-drive voltage. This will put some constraints on the 
gate-driver. So, tradeoffs need to be made. I'd suggest picking a device 
with a low-enough Rds(on) such that it's power-dissipation does not cause 
significant heat-rise with a small heat-sink.

Using the tiny PCB-mounted devices is tricky for thermal reasons; I prefer 
TO-220 packages or similar.

There's a lot of really good info in the TI datasheet, appnote, and the 
IRF7807 datasheet. Once you read thru all of that you will have a good 
understanding. Just be aware the IRF7807 wont give you nixie-level voltages 
as t's rated for 30V max.

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